How can polytetrafluoroethylene stick to frying pans?

Here's what you've said so far:

From Ben Peters, November 6, 1998:
All the teflon frying pans I have seen are aluminum. The reason aluminum doesn't rust is that a thin layer of aluminum oxide forms on the surface of the metal. If you blended, copolymerized, or better yet were able to sythesize a block copolymer of TFE with a monomer that would hydrogen bond with the aluminum oxide ie. vinyl alcohol. You could pour a hot homogenous mixutre on the aluminium and upon cooling the mix would microphase seperate with the OH moeity concentrating on the metal surface and the CF migrating away from it. H-bonds are stronger than Van Der Waals interactions and the in the case of the copolymerizations the two components are chemically bonded. Even in the blend case entropy would probably be such that chains of CF would remain imbeded in chains of OH. It is hard enough to get small molecules to crystalize quantitatively. Consequently, once the mixture has "set-up" on the metal surface heating would not return the coating to homogenaity. As each phase of the polymer would rather stay separated OH-Al CF-not Al or OH than recombine OH-CF.
That is a very good idea, using the aluminum's own corrosion to aid in binding PTFE to the pan. A copolymer would probably work better than the blend. The blend woul dindeed tend to phase separate. You see, polymers don't gain very much entropy on blending, so they don't blend unless there are strong enthalpic interactions. There would not be such enthalpic interaction in the blend here, so PTFE and polyvinyl alcohol would probably phase separate. Copolymers are a better way to go. Random copolymers might actually work better than block copolymers because even "random" copolymers have blocky segments. Random copolymers have more blocks per chain than true block copolymers, which means there are more ties between the frying pan surface and the polymer layer.

Thanks for your idea, Ben!

From Adam Lounds, July 30, 1998:
1. Prepare pan by sandblasting.

2. Add primer layer, eg [-CF2-C(something sticky)2-]n. PTFE will stick to itself quite well (PTFE tape). My guess is that if you get a polymer that can present a layer of fluorinated polymer to PTFE, PTFE will stick to that as well.

3. Spray/pour over PTFE. It will stick to the primer layer.

We know that the process involves grit-blasting or etching from DuPont themselves. I suspect that this helps with the sticking of the primer layer.

I think it's quite clear, Adam, from reading the reference you cite that some sort of compatiblizing third component is used to make PTFE bond to frying pans when the DuPont source says:
"Although we can't divulge the ingredients of our products, we can say that a blend of an organic mix and PTFE (polytetrafluoroethylene) allows this process to take place."
A semi-fluorinated homopolymer as you suggest is a good possibility. Another candidate would be a copolymner of tetrafluoroethylene with some other monomer.

However, as far as surface etching, the DuPont source says they used such techniques at one time, but they go on to say:

"DuPont has recently introduced new patented technology in the primer which allows TEFLON(R), SILVERSTONE(R), and SILVERSTONE(R) Xtra to be applied over a smooth substrate."
So it seems they've figured a way to put this stuff on without etching. Does anyone have any ideas about what that might be?

I must stress here that the surface etching would be used not to make PTFE stick to the pan, but to make the compatibilzer stick to the pan. This in turn would allow PTFE to be bonded indirectly to the frying pan. But without a compatibilier, etching would only increase the repulsion between the metal pan and the PTFE.

Thanks for your response, Adam!

From Craig Fuller, March 18, 1998:
I think that the surface of the PTFE is etched with sodium to remove some of the fluorine atoms and produce free radicals. The polymer is then able to bond to the metal at these sites.

Good thinking, Craig! Sodium metal is one of few things that will actually react with polytertrafluoroethylene. Thanks for your response!

From Jeff:

From what I have heard you use a composition of PTFE with a strong oxidizer (Chromium tri-oxide). The first layer is sprayed onto the pan with a high percentage of CrO3 and each subsequent application you use a lower percentage of CrO3. When you reach your final layer you have an almost pure layer of PTFE with a color and metallic flakes added to conceal the previous layers.

Sounds pretty cool to me!

From Dietrich McCall:

The metal of the frying pan must have texture where the teflon will be able to stick. You must melt the teflon at very high temperatures. Once the teflon is melted you would let it "settle" into the ridges on the frying pan.

Roughening surfaces helps when you want an adhesive to stick to a surface. The reason this happens is that roughening the surface increases the total area of the two surfaces that will be in contact. Interactive forces between two surfaces increase with increasing surface area of contact. What this means is that if two surfaces are attracted to each other, they will be attracted even more strongly when the surfaces are roughened.

However, if the forces are repulsive, as is the case with PTFE and steel, roughening thesurface will only increase the force oftherepulsion between the two surfaces. So roughening the frying pan will make PTFE stick even worse than it does on a smooth surface.

Thanks for you question, Dietrich!

From Carl:

Teflon is mixed with a polymer that attracts PTFE. This substrate is soft and has a single carbon bond with it's host (the aluminum alloy of the pan) The weakness of the bond is demonstrated by how easy it is to scratch through a teflon pan.

So how do they bond Teflon to rainjackets? Do they use similar bonds? Or is it a true laminate?

Blending PTFE with a second polymer, and covalently bonding the second polymer to the pan sounds like a good idea to me. Of course, finding a polymer that will blend with PTFE is going ot be difficult. As for Gore-Tex, it's my understanding that it is a laminate, and they don't really need to bind it to the other materials.

Thanks for your input, Carl!

From Douglass Campbell:

Defluorinate the surface of the PTFE being bonded to the other surface (ie. aluminum or any other surface). Increase suface contact area by putting some texture in the surface to be bonded the teflon.
The idea of having a material that is fluorinated on the top, but not fluorinated on the bottom, the side that has to stick to the pan, is a good one. However there's a problem in that it's hard to defluorinate PTFE. Carbon-fluorine bonds are too strong. A good idea might be to work from the other direction: coat the pan with polyethylene, and then fluorinate the top side of the coating. This can be done by simply blowing F2 gas over the surface of the polyethylene. Although you might not want to use toxic fluorine gas to make a cooking pan, this method is used to give polyethylene tubing a PTFE coating on the inside.

As for roughening the surface to increase contact area, that might be a good idea, if the metal in the pan and PTFE liked each other. Adhesive forces are proportional to the amount of surface area in contact. However, repulsive forces are also proportional to the amount of surface area in contact. If we roughen the surface, we increase the surface area, and therefore we also increase the repulsive force between the PTFE and the frying pan.

Thanks for your response, Douglass!

From Rafael Pinto:

Sometimes when I'm frying eggs or making a cheeseburger, either the egg white or the cheese sticks pretty hard on the frying pan. Maybe some protein like albumin or casein can make PTFE stick to something. Another strange idea I got is: What if you make the PTFE coating a little porous,...We can stick with another polymer thru the "holes". Sorry if it sounds [like] stupidity, but it's an idea from an electronic engineer after lunch!

Using holes to bolt the PTFE down with another material isn't a bad idea, as far as I can tell. You might even make it easier by having the steel of the frying pan itself stick through the holes to bolt down the PTFE. As for using proteins to glue PTFE, they probably couldn't handle the temperatures that are found right on the surface of the frying pan. It would char just like a hamburger patty does when you forget to flip it. Usually when stuff sticks to a ptfe coated pan, it's because the coating has been scrubbed off in washing. You have to be careful with those pans because scouring pads can scrape PTFE right off.

Thanks for your answer, Rafael!

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